1. Field of the Invention
The present invention relates generally to consumer device audio power output stages, and more specifically, to a circuit that selects the efficiency of a charge-pump power supply within an audio amplifier integrated circuit based on the amplifier's operating environment and/or process position of the integrated circuit.
2. Background of the Invention
In battery-operated consumer audio devices, the power amplifier output, which is generally utilized to drive a pair of headphones or a speaker, can be the primary consumer of battery power. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, since the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s).
In order to provide a changeable power supply voltage to such a power amplifier, a charge pump power supply may be used, such as that disclosed in the above-incorporated U.S. Patent Application, in which an indication of the signal level at the output of the circuit is used to control the power supply voltage. The above-described topology will raise the efficiency of the audio amplifier, in general, as long as periods of low signal level are present in the audio source. Since the minimum voltage drop across each output transistor of the amplifier is relatively large for small signal levels, the power supply voltage supplied to the power amplifier can be reduced during times that the signal level is low. The power supply voltage can then be increased just before additional voltage is needed at the amplifier power supply, in order to avoid clipping at the output of the power amplifier.
However, signal level alone does not provide a complete indication of characteristics of the audio amplifier and the circuit environment. For example, the impedance of various transducers that may be connected to the amplifier varies, causing a variation of power level in the transducer for the same output voltage. Further, the supply voltage provided to the integrated circuit may vary from application-to-application or in a given application, which may dictate lower charge-pump device resistances in order to provide a required amplifier operating voltage. Also, process variation of the integrated circuit and the operating temperature affect both the on-resistance of the charge-pump devices and the output resistance of the audio amplifier. Therefore, the amplifier and charge pump must be designed robustly in order to maintain operation without clipping across the specified operating temperature range and a range of process variation for a given production yield. The robustness of the design compromises efficiency, as the amplifier power supply voltage required to reproduce a given audio signal level without clipping increases as the voltage drop in the amplifier power stage increases and that voltage drop is dependent on the environmental and process-dependent factors mentioned above.
Therefore, it would be desirable to provide a charge-pump power supply that supplies power to an audio power amplifier circuit for a consumer audio device, in which the efficiency of the amplifier is improved by changing a power supply voltage supplied to the amplifier, and in which the decision to change the power supply voltage takes into account the operating environment and/or process variation of an integrated circuit including the audio power amplifier.